The ultrastructure of protein hormone–producing cells is characterized by abundant rough endoplasmic reticulum and Golgi membranes and the presence of secretory vesicles •Fig. 38.3 Acircadianpacemakerdirectsnumerousendocrineand bodyfunctions,eachwithitsowndailyschedule.Thenighttimerise inplasmamelatoninmaymediatecertainothercircadianpatterns.ACTH,adrenocorticotropichormone.(DatafromSchwartzWJ.Adv Intern Med. 1994;38:81.) in the membrane of the endoplasmic reticulum located on the cisternal (i.e., inner) surface of the endoplasmic reticular membrane. Removal of the signal peptide by a signal peptidase generates a hormone or prohormone, which is then transported from the cisternae of the endoplasmic reticulum to the Golgi apparatus, where it is packaged into (
Fig. 38.6
Protein/peptide hormones are soluble in body fluids and, with the notable exceptions of IGFs and growth hormone, circulate in blood predominantly in an unbound form and therefore have short biological half-lives. Protein hormones are removed from blood primarily by endocytosis and lysosomal degradation of hormone-receptor complexes (see the section “
Cellular Responses to Hormones”). Many protein hormones are small enough to appear in urine in a physiologically active form. For example, follicle-stimulating hormone and luteinizing hormone are present in urine.
Fig. 38.5 Schematic Representation of Peptide Hormone Synthesis. Inthenucleus,theprimarygenetranscript,amessengerRNAprecursormolecule,undergoesexcisionofintrons,splicingofexons,cappingofthe5′ end,andadditionofpolyadenylation(poly-A)atthe3′ end.TheresultantmaturemessengerRNAentersthecytoplasm,whereitdirectsthesynthesisofapreprohormonepeptidesequenceonribosomes.Inthisprocess,theN-terminussignalisremoved,andtheresultantprohormoneistransferredvectoriallyintotheendoplasmicreticulum.TheprohormoneundergoesfurtherprocessingandpackagingintheGolgiapparatus.Afterfinalcleavageoftheprohormonewithinthegranules,theycontainthehormoneandcopeptidesreadyforsecretionbyexocytosis.NH2,amidogen.
Fig. 38.6 Ultrastructure of a Protein Hormone–Producing Cell. Notethepresenceofsecretoryvesiclesandroughendoplasmicreticulumintheproteinhormone–secretingcell.(FromKierszenbaumAL.Histology and Cell Biology: An Introduction to Pathology. 2nded.Philadelphia:Mosby;2007.) •Fig. 38.7 Chemicalstructuresofcatecholamines.
Proteins/peptides are readily digested in the gastrointestinal tract if administered orally. Hence they must be administered by injection or, in the case of small peptides, through a mucous membrane (sublingually or intranasally). Because proteins/peptides do not cross cell membranes readily, they signal through membrane receptors (see
Catecholamines are synthesized by the adrenal medulla and neurons and include norepinephrine, epinephrine, and dopamine (